[DOC] Document ReleaseEnv memory leak limitation before v1.0

[tazarov]

Description

The current implementation has a known memory leak in DestroyEnvironment() because ReleaseEnv is disabled (see issue #20). This limitation must be prominently documented in user-facing documentation before any production use.

Background

From PR #15 review:

The ReleaseEnv function is currently disabled, meaning every environment creation leaks memory. This is acceptable for short-lived processes but CRITICAL for long-running applications.

Required Documentation Updates

1. README.md - Add Limitations Section

Add a prominent "Known Limitations" or "Current Status" section near the top:

## ⚠️ Current Limitations

**Not Production Ready**: This library is under active development and has known limitations:

- **Memory Leak**: The current implementation leaks memory on `DestroyEnvironment()` due to incomplete ORT API struct mapping (see #20)
  - **Impact**: Not suitable for long-running applications that repeatedly create/destroy environments
  - **Acceptable for**: Short-lived processes, testing, development
  - **Status**: Tracked in #20, must be fixed before v1.0

See [LIMITATIONS.md](LIMITATIONS.md) for full details.

2. Create LIMITATIONS.md

Document all known limitations with:

• Clear description of each issue
• Impact assessment
• Workarounds if any
• Timeline for fixes
• Link to tracking issue

3. Update environment.go godoc

Add warning to InitializeEnvironment() documentation:

// InitializeEnvironment initializes the ONNX Runtime environment.
//
// WARNING: Current implementation has a memory leak on DestroyEnvironment().
// Not recommended for long-running applications that repeatedly init/destroy.
// See: https://github.com/amikos-tech/pure-onnx/issues/20
//
// This function uses reference counting, so multiple calls are safe.

4. Update TESTING.md

Document that integration tests should verify cleanup once #20 is fixed.

Acceptance Criteria

• README.md has prominent limitations section
• LIMITATIONS.md created with full details
• InitializeEnvironment() godoc includes warning
• Documentation clearly states "not production ready"
• Links to issue [TST] Fix OrtApi struct layout to enable proper ReleaseEnv cleanup #20 for tracking

Priority

High - Must be done before v1.0 or any production recommendation

Related Issues

• [TST] Fix OrtApi struct layout to enable proper ReleaseEnv cleanup #20 - Fix OrtApi struct layout to enable proper ReleaseEnv cleanup